In an astrophysical context, the photonsphere (a
compact null hypersurface around the central black hole) determines the optical appearance of a
compact collapsing object as seen by external asymptotic observers [1]}, [2]}.
Likewise, the intriguing general relativistic phenomenon of strong
gravitational lensing by black holes is closely related to the presence of
null circular geodesics in these curved spacetimes [3]}.
